Well logging



Feb, i3, 1951 K. c. CRUMRINE WELL LOGGING 5 Sheets-Sheet l Filed July 7, 1947 Q Q@ www@ E l Il P\l I I I I l l I l 1| Il w w 1 m f N T M @u ww M w m m o V l N fw fw T I C A m M YM w m M 5 5; N M TE Y mw m. r nl ao@ g3g M nww l I I l I aw 1l u 6C mm u A w ww n LS w .@m

Feb', 139 Q K, c, QRUMRBNE I 2,54L341 WELL LOGGING Filed July 7, 1947 3 Sheets-Sheet 2 FAST NEUTON-SLOW NEUTRON GAMMA RAV ELECTRICAL 06S L06 LOGS (AFTER :EMEA/HMG) rae-Fons CEMENT/Iva; /NrE/vs/rv /NcnEAsEs /NrE/vs/rv//vcnEAsEs /NrE/vs/Tv/NcnL-Asss sfu- PorE/vr/AL @Es/sr/wrv m/Af/ (BEFORE CEMENT/NG) ,9u/vz ,u/LL/voL rs OHMS mim /ooo AFTER I-l--HII I'l''l'x CEMENT/Ns 2o 0 4 f2 /6 102 ser nu .s T

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INVENTOR. KENNETH 06m/MEINE A T TORNEV Feb. 13, 1951 K. c. cRuMRlNE WELL LOGGING 3 Sheets-Sheet 3 Filed July 7, 1947 ,IHIIIIIVWIIIIJIIJIillllllllllll.

R N W mm rw m N m K Y DD mm uw m, ww uw ATTORNEY tify formations penetrated;

Patented Feb. 13, v-1951 WELL LOGGING Kenneth C. Crumrine, Tulsa, Okla., assigner to The Texas Company,A New York, N. Y., a corporation of Delaware lApplication July 7, 1947', Serial No. 759,368

This invention is concerned' with well logging t r 'andA provides improved apparatus and technique for this purpose.

In wells drilled for either petroleum explorationor production, it is important to identify the position and character of thestrataencountered, and also to locate the level at which various installations are made, for example, the position of concrete introduced for purposes of water shut off.` Strata penetrated by the well may be identied by core samplesr taken during the drilling operation, but coring is an expensive procedure which slows down drilling operations, and cor.- ing,"` of course, gives no information with respect tothe location of plugs, etc. located outside the welll bore proper after d-rilling. In consequence, various logging methods have been employed .to lobtain more cheaply the information tov be gained by coring or to obtain additional information not thus obtainable. All of the methods involve traversing the bore with an exploring tool and measuring a characteristic which variesA from one formation to another. One method makes use of variations.- in electrical. resistivity to idenanother employs `Variations in self potential; and. another measures changes in gamma radiations naturally emitted by the formations. Each methodis superior in certain aspects. Thus a shale formation may give a pronounced response in one method and be substantially' undetected by a second method. which isk superior tothe rst in distinguishing between two different sandstones. Consequently, several methods may be employed in the same well, the sum of the information collected being greater than its parts. However, in some cases, the aggregate information is still inadequate, especially whenY efforts are made to correlate logs from different wells, as for ex- .ample when it is: desired to determine whether a. formation occurring in one well is also encountered in another and if so, how thick it is.

We have discovered that useful information can be obtained. by traversing a bore with a source of fast neutrons accompanied by a detector of slown neutrons and. determining variationsin the intensity of the slow neutrons detected along the bore. An exploring tool containing a source of fast. neutrons and a detector of slow neutrons is lowered into a well. The fast moving neutrons experience nuclear collisions in the region surrounding'the source (L e. in the exploring, toolitself, in the.: uid, if any, in the bore, innthex casina, if any, and ini the neighboring rock formation) .and are thereby 2 scattered and slowedl down'. Some of the resulting slow neutrons are thus scattered back to.A the detector, which measures their intensity, the log being a record. of this intensity as a function of well depth. Changes in the intensity, which may be as much as 10% or 20% of the total intensity; serve to identify the formations traversedk and may be employed alone or in conjunction' with .other logging methods to gain information with respect to well conditions. I;

The slow neutron intensity at the detector is a complicated function of theiin-teraction properties of neutrons and-the nuclei of atomswith whichl they collide. in the' neighborhood of the exploring tool. These interaction properties vary widely among the elements. Nevertheless, the above described method, which is sometimes referred to hereinafter as fast neutron-slow neutron logging, does yield useful informationv in the form of reproducible differences in slow neu*- tron intensity between diierent kinds of formations; and by logging: wells in which information is available regarding the formations penetrated and other well conditions, it.` has been possible to accumulate empirical data that serve to identify the unknown conditions in other wells similarly logged. i Fast neutron-slow neutron logs have been obtained inA accordance with the method of invention in a number of bore holes where the conditions are approximately known from core samples and logs obtainedy by other methods, and on the basis of the information obtained, it canl be; con eluded that fast. neutron-slow neutron logs:

A.v are correlatable. from one bore to another and so may be used to4 identify corresponding conditions in each;

B. are correlatable with certain gamma ray logs but are sufficiently different from others to justify running both` typesv in some cases, es.;- pecially when it is desirable tov distinguish between limeand anhydritey formations; Y C. are less readily matchedwith. electrical logs and give differ-.ent information; g

D. show greater changes. in neutron intensity within formations containing lime than in otherformations; V .y

E. are adversely laffected by iron` employed to case the bore, to the extent that two stringsof casing in some cases affords too much shielding yof the detector;v and I F. are affected by well flu-ids to the extentth'at insome holes it is desirable, in.` order tof obtain optimum results, toA employ an exploring tool of smaller cross' sectionand in others to enlarge the cross section of the tool by placing a sleeve, preferably one of aluminum, on it; thereby displacing well iluid in the neighborhood of the tool.

In the practice of the invention, any suitable source of fast neutrons may be employed. Most practical sources, however, also emit gamma rays, and a proportional radiation counter (which must be especially sensitized for the detection of slow neutrons) is also sensitive to gamma rays or to anything else capable of producing electrons within the active volume of the counter. Thus gamma rays tend to obscure the slow neutron effect. In one of the aspects of the invention, the detector is so constructed that the amplitude of at least some of the pulses attributable to the slow neutrons is greater than the maximum amplitude of the pulses due to gamma rays, and the eiect of the gamma rays is 'minimized Vby suppressing in the output of the counter pulses of the amplitude of those attributable to gamma rays,leaving a response due solely to slow neutrons.

In fessence, the apparatus of the invention comprises an exploring tool containing a source of fast neutrons and a detector of slow neutrons .(say a proportional counter), means for traversing the bore hole with the tool and for energizing the detector, and means for measuring the response of the latter. In its preferred form, the apparatus includes a source of fast neutrons which emits a low intensity of gamma rays. However, sources which emit fast neutrons and intense gamma ray beams can also be used together with a radiation detector disposed adjacent the source and sensitive to both slow neutrons and gamma rays, but so constructed that pulses due to at least some slow neutrons have an amplitude greater than any pulses in the detector due to gamma rays, means for suppressing in the output of the detector the pulses having an amplitude up to and including the maximum amplitude of pulses due to gamma rays, and means for measuring the outputl of pulses not so suppressed. Y jf A preferred neutron source is a metall capsule containing about 200 milligramsY of radium and several grams of beryllium. The radium emits alpha and gamma rays. The gamma yrays (photons) escape from the' capsula The alpha particles have small penetrating power and do not escape. However, some of them are captured by beryllium nuclei with emission of vneue trons having a kinetic energy of several million electron volts, which leave the source at high velocity and penetrate the capsule walls, the-well bore and the surrounding casing and rock formations. In so doing, these fast neutrons experience collisions with the nuclei of atoms of the surrounding materials and lose energy. Some are scattered back to the counter.

i The neutron counter is constructed to capture and detect the slow-neutrons which encounter it, i. e. those scattered back into its active volume. Preferably it comprises a plurality of proportional counters arranged around the fast neu- 'tron source. Each counter comprises a tubular cathode through which a thin anode wire passes. rv Anode and cathode are disposed in a suitable gaseous atmosphere, say methane, retained in '-an envelope. The anode ismaintained at apositive potential with respect to the cathode. A high resistance is placed in the circuit, and the potential difference between the two electrodes is maintained below that at which a discharge takes place. If a particle capable of ionizing frs the gas enters the active volume of the counter, it will trigger, with resultant oW of a small current. This causes a voltage drop across the resistance which is amplied and recorded.

Since slow neutrons produce no direct ionization, the counter must besensitized, as by incorporating in it an element which will capture slow neutrons with emission of alpha particles, which in turn cause ionization within the counter. Preferably the counter is sensitized by coating the inside of its cathode with a boron compound, although it may also be sensitized by operating it in the presence of a gaseous boron compound. The active isotope of boron, as well as those of other elements such as lithium, have the property of `capturing a slow neutron with `the emission, of alpha particles and an energy of several million electron volts. The alpha particle produced has considerably less penetrating power than an electron having the same energy, because of the greater charge and mass of the alpha particle. Moreover the energy of some alpha particles is expended inthe coating prior to entry into the active volume.

The counter employed in the preferred prac tice of the invention is so designed that only a fraction` of the energy entering inthe formof gamma rays is expended within the active volurne,V while a much higher proportion ofV the energy of alpha rays is so expended. This may be accomplished by employing tubes of certain dimensions and by properly" selecting the 'pressure and kind of gas employed within the active volume.` As a Vresult,`sorne of the alpha particles produced from the slow neutrons (i. e.'those which have not dissipated too much'of their energy in escaping into the active volume from the coating) give bigger pulses than the biggest pulses produced from the gamma rays.

The amplier employed in the preferred'form of apparatus to receive the pulses from the counter operates as a discriminator and is so arranged that it passes only pulses having a' pre'- determined amplitude, this amplitude being selected sothat the small pulses dueto gamma rays are suppressed. A v Bearing the foregoing principles in mind,Y the operation of the preferred device for astneutron-slow neutron logging 'is as follows:V

An exploring head containingthe slow neutron counter and the fast 'neutron source is `lowered down a bore hole on a logging cable 'which' supeplies direct current powerto the Vequipment in the head and also 'transmits' pulses from the counter back Vto surface equipment. Also in'- cluded in the head is a vacuum tube 'amplierdiscriminator of the type' mentioned abovef It receives pulses froin'the counter due to the acition of gamma rays and slow neutrons therein, and' separates from the totality of the pulses received some of those which are dueto' slow neutrons alone. These it impresses on the logging cable.' 11

In order to avoid diiiiculties 'inherently at'- tendant upon impressing a'high D. C. voltage on the logging cab1e,.it is preferable toV include vin the' exploring head a vacuum tube circuit, called a high voltage supply,'which steps `up 'the voltage received from the cable and ysup'- plies the necessary high voltage current toi the detector'or counter. 5 :i Lastly, the exploring head preferably Vcontains an automatic volume control adapted Ato make the pulse -rate independent of changes in 'Pulses sent -to the surface' throughthe cable aredamplied. and measured in t conventional equipment and preferably are recorded as a function of well depth, thus givinga log.

l" To summarize the actionk of the preferred apparatus of the invention, fast neutrons given off bythe neutron source penetrate the formations surrounding the logging head with the result that some are scattered back as slow neutrons to the counter. The slow neutrons strike a sensitive element, say boron, in the counter which gives off alpha particles of high energy but small penetration. Some of these escape with practically undiminished energy into the active volume between the anode and cathode of vthecounter.l Atthe same time electronsrdue to'gamma'rays enter the active-volume of the counter, and both the alpha particles and the electrons produce pulses in the output of the counter? The construction and adjustment of the counter is such, however, that some alpha rays expend more of their energy in the counter than` do gamma rays so that the pulses due to the alpha rays, that have not lost excessive energy' in entering the active volume, are of greater amplitude than any due to gamma rays. The amplifier suppresses all pulses having an :amplitude as great as or less than the greatest pulse due to gamma rays, and transmits the remaining pulses, which are due solely Ito alpha rays produced from slow neutrons, to thev amplifying and measuring equipment at the surface.' vThese vand other aspects ofthe invention will be .more thoroughly understood in the light of .the following detailed description taken in conjunction with the accompanying drawings in ".WhCh:

Fig.1 is a wiring diagram of a preferred form ofthe logging apparatus;

Fig, 2 gives a comparison of .logs taken in the .same well by the fast neutron-'slow neutron technique of the invention, by a conventional gamma ray method, by the self potential electrical method, and by the resistivity method;

Fig. 3.-is a section-'taken alongthe line 3-3of Fig. 4 through a portion of a detector head of the invention, including a source of fast neutrons and a multiple proportional counter sensitized forI the detection of slow neutrons;

., YFigA is a cross section taken throughthe ap- .paratus of Fig. 3 along the line 4 4; and

Fig. 5 is a fragmentary longitudinal section 4:through the apparatus of Figs. 3 and 4,y talren along the line5-'5 of Fig. 4. 1

, .l The apparatus in general Referring to Fig. 1, the apparatus in essence 'o'omprises' a logging cable I0, a counter ll for detec'zting slow neutrons alongl with .gamma rays,

-asignal transmitter or amplier-discriminator f'channel I2 adapted vto suppress pulses other 'than those due to some slow neutrons, a coun- -tervoltag'e supply I3 and an'autorn'atic volume control III.v v

' lThe logging cable I supplies direct current to VLthe circuitof'Fig. l from equipment (not shown) *at the surface of the ground and also transmits fcurrent 'pulses from the circuit of Fig. 1 'to amplifying andl recording equipment (not shown), also disposed on the surface. Thus pulses originating in the-neutron'counter I I are transmitted to the logging cable through the amplifier-dis- 'igcriminatorIL which separates from thetotality :toithe ppulsesvreceived some of those whichare due. to slow neutrons alone. AThe high voltage' Power and signal transmission In Fig. l, the shield of the cable Ill is con sidered to be at ground or zero potential, and the conductor of the cable is maintained at about 250 volts above ground by a primary/.power source (not shown). A direct current of about .6 ampere flows from the conductor of the cable through variouspaths in the unit of Fig. l to ground. One such path includes the heating elements of certain vacuum tubes of the. apJ paratus as shown on the extreme right of the drawing, the various elements being designated by the same numerals as those employed for the respective tubes hereinafter, but with the prefix H. Another path includes two inductances- I5, IS and that portion of the circuit which is the high voltage supply I3 of the counter. A third path contains a high resistance 'I'l (say 22,500 ohms) from which various plate, screen and cathode voltagesupplies for vacuum tubes are tapped olf as shown.

As indicated above, the cable receives current pulses at varying rates from the counter via the discriminator I2 and transmits them to the sure face equipment which amplies, equalizes, in-f tegrates and records them in a conventional manner as a function of depth of the well being logged. y

The counter voltage supply I3 comprises fa vacuum tube circuit containing tubes i8, IS, 20., 2|, 22, 23, 24 connected as shown. The counter voltage supply receives current at relatively low voltage from the logging cable and modifies it to produce a stable and filtered potential of about 1,000 volts positive with respect to ground, this voltage supply being impressed across the counter-.1 :n:

Tubes 20, 2I, 22, 23 "are heater type tubes: provided with heating ele-ments energized as indicated hereinbefore by that portion of the circuit shown at the extreme right of Fig. l. In the preferred structure, the tubes of the countervoltage supply are ofthe following types: j

la 1H4G Tube Tube I9 801e` Tube 20 35A5 Tube 2l 6J5 Tube 22 'IC7 Tube 23 -35A5 Tube 24 VR'105 To consider the circuit in further detail, the

primary winding of a transformer l25 in series with a condenser 26 forms an oscillator circuit, oscillation thereof at a frequency of about 500 cycles per second being maintained by the tube 2I. This signal frequency is applied to the tubes 20, 23, which are identical, as indicated above, and operate in a push-pull power amplifier .,ei-rcuit. A transformer 21 transmits power' from this amplifier circuit to the filaments of the tubes I8 and I9 and impresses a potential of about 2,000 volts peak on the plate of the tube I 9, which acts as a rectifier. A capacitance 2Br to ground between the tubes, I8 and I9 partially smooths the rectified Wave form: Part: of the' rectified current at high voltage passes through the tube I8 which in effectA constitutes a variable resistance controlled by the tubes22, 24. Thus tubes I8', 22, 24 together with their'associated circuit elements form av voltage stabilizer and lter which maintains the potential at the filament of the tube I8 essentially constant at about 1,000 volts above ground or atsuch other Value as may be selected. The Wave form is further smoothed by a resistance-capacitance ilterV through which the high voltage supply is connected .to the anode of the counter IIVvia an additional resistance 3I. The cathode of the counter is connected to ground as shown.

V Amplijerv 'and discriminator Attention is .directed .to themain amplier and discriminator channel I2 which contains vacuum tubes. 32, 33, 34, 3.5,V 3S, 3l. Negativeyvoltage pulses from the anode of the counter are impressed upon the discriminator through the grid of the -tube 32 (which preferably is of type 1852) through a blocking condenser 38. The tubes 33,

35 and 3E are likewise of this type, while tube 33 is of type 1853 and tube 3l is of type 50L6.

After amplification by the first tWo stages, the pulses appear at the grid of the tube 34 with vthe original pulse size distribution. The tube 3d has a remote cut-orf characteristic anda compressing effect upon the pulse size distribution. At the grid of the next tube 35, which is biased beyond cut-off, a selection from the pulse sizes is effected, all pulses smaller than a certain Asize being suppressed bythis stage so that there is transmitted to the grid of the next tube 36-only the larger sizes.A Proper adjustment of the discriminator assures that those pulses whichare transmitted are practically all caused vby slow neutrons with practically none dueto 'gamma s Automatic volume control That portion of the circuit of Fig. l constituting the automatic volume control I4 comprises vacuum tubes 4I, 42, 43, M, [I5 with the associ; ated circuitvelements shown. Tubes lll, 42, and are of type 1852 and tubes 43 an d @.4 are.. of type GHG. v Y l n The output'of the tube 33 in the discriminatorv-amplifier I2 contains unequalized pulses due to neutrons and gamma rays. rl'hese are impressed 4upon the grid ofl the tube 4I theautomatic volume control. After amplication by two stages represented by the tubes 4I and ft2, the pulses are more -or less equalized and are im.- pressed upon the cathode of thetube 3,which acts as a rectifier. rectifier tube d3 are integrated by a resistancecapacitance network 116 andV a negative voltage for use as automaticvolume control bias is Amade available at point il in the circuit. Al relatively small proportion of this bias voltage is due to neutron pulses because, in the output of the vvcounter, for reasons expressed hereafter, the

gamma ray pulses ordinarily outnumber the neutron pulses at least ten to one and probably'more.

'A--correction is made, however.' to offset this'co'n- The output pulses from the tribution'due to -neutronipulses by'fimpressingithe output of the tube 35 of the amplifier-discrinfr-J inator *on-the gridof the' tube 45 in the AVCV cire cuit. The output of the tube 35 in the amplifier-' discriminator is representative only vof :neutron pulses. After rectification` by thei rectierjtube 44 and integration bythe resistance-capacitance network, these neutron pulses supply alpositive voltage to the point 41 of the network.;y Such positive voltage can be 'adjusted in magnitude to offset the previously'. described negative contribue` tion from the neutron pulses. 'The net AVC vcilt age at the point 4.1 is therefore almost entirely due to gammaray pulses, a'nd'inA Aconsequence it is invariant withz respect to 'changes in the., neu-e tron pulse rate from the counterfwhich are, due to changes in environment, `say to changes inrock formation encountered. 1 In short, anautomatic control voltage is developed at the-point v,4l Awhich cannotV affect thesensitivity of theequipment to the neutron density changes at the counter `for the measurement of which the equipment is de:` signed. The net AVC voltage does not depend upon the measured quantity.

By way of explanation, let it be considered that the grids of the tubes in the main .amplifier-.dise criminator channel I2 are connected to a fixed negative potential of say 5 volts instead of to the AVC bias point 41j as shown in Fig. 1. Then. let the counter voltage be increased. The amplitude of all pulsesiyielded bythe rcounter increases, and their number increases because .the pulse size must still be graded downward to zero. There will then be a greater. Vnumber of' larger-:.:pulse amplitudes at the grid of the tube 35 for" overjriding the cut-01T bias'of this tube, andconse'- quently the pulse rate delivered through this tube to the cable will be increased. It willbe plain that a counter and amplifier so constructed have no plateau as a function of counter .voltages- However, thissan'le increase in counter voltage has caused delivery of agreater number of pulses to the AVC channel, and the bias' voltage 'at point 41 has therefore become more negative. "Solif nsteadiof the fixed bias of 5 volts appliedito-.the 'tubes 'of the amplifier'-discriminator, there. be applied the AVC bias voltager rom'the poi-ntflt'I -(as is actually the'case in-FigL 1) the gain-of.y the tubes 32,' 33,v and 34 vvillv be decreased and tube 35 will become'biased stillfurth'er beyondcut' 01T. *A plateau may thus be achieved by th'implier, counter andV automatic volume--control incombination. .5

To take another case, if the cathode emission in the several tubes be increased as a resultof an increase in heater currentor as a result of an increase in temperature in the bore hole, the gain ,of the tubes 32, 33, 34 with fixed bias;of-Say 5 --volts Will be increased, and more pulses can over.-

ride the cut-off bias Vof the tube 35 to increase the pulse rate at the'cable. But the same factors which cause an increase in the cathode emission likewise increase thegain Yof the stages in the automatic volume-control Ill'so that the bias voltage at the point 41 will become morenegaf tive. If this'bias be employed instead of fixed bias in the amplifier-discriminatorchanneLfthe gain of the tubes 32, 33,-34 'will-"be more constant and the pulserate delivered tothe cable-,will-,be invariant with respect to changes in Acathode emission.

Back wave elimination As indicated` above,the tube '43 in the AVC 'channel "I4 is of the--duou1oae1type.-HaIf cette tube -sfconnected `trl-elfe,cth/ely eliminate positiveV pulses onthe` gridofthe tube 34v in the am'- plierfdiscrirninator channel, because on the latter grid only-negative pulses have significance. Similar use is made of; half of the Aduodiode tube 4 4Jjof the vAVQcha-nnel, which is connected to the grid of the-tube 4.1:.. p v

l Decouplzng )titers The capacitance 39 (of say .5 microfarad) is employed l.as described hereinbefore to pass pulses fromthe cathode of the tube 31 to the cableand to .prevent the D'. C J. power supply voltage from aiecting this cathode.- The air core coilv linoffers a high impedance tothe pulses but avery Alow impedance to the C.v supply current rom the cable.. Thev chokes 'l 5 and I6 A have iron cores andpresent relatively lou/ impedance tothe iiow the primary supply current and effectively prevent the 500cycle per vsecond alternating current, produced by'the oscillator, from affecting the B+ supply or ythe heaters of thevacuum tubes or' the logging cable. As indicated in the drawing, liberal use of resistance-capacitance de-coupling filters is employed in the plate circuits of many of the stages.

The counter The counter Il shown d-iagrammatially in Fig. ljis shown in detailin Figs. 3, land 5 The apparatus employs a neutron source in the form of about 200 miligrams ,of radium and severalgrams of beryllium, -inclosed in asealedmetal cylinder 0. This capsule is held -by a-irictionv grip in a source holder A5I 1 disposed on the central axis of the counter.- This counter is an elongated cylinder 52 having van insulator 53 mounted concentrically on one end plate 54 and with a source hole 55 and a gas exhaust tube 59 inthe other end 131211958. p-

yThe sourceholder is-threaded into-a reducer 56, t he outerl threads of 4which t the inside threads of the source hole. Thus the source capsule is disposed in a center tube l51 andis centrally located with respect to the counter as a-whole. The exhaust tube is fastened to and passes through the end-plate 58. i rIhis exhaust tubeis crimped orf after the equipment is assembled and iilled withan appropriate atmosphere, and nally sealed with soft solder.-

Eight cathode tubes A, 60B, 60D, 60E, BUF, 60G,160H, 68.] are -spaced symmetrically in th'e annulus within the cylinder around the center tube, Each isabout 24 inches long and has an inside diameterof about 0. inch and an outside diameter of 0.25- inch. fhese tubes are metallic, and -each is; coatedwithboron carbide powder heldin-a binder .-ofgnitro-cellulosecement. The boron carbide -powder] employedv shouldbe lof smallparticle size, i;v-e.- thev maximum dimension of the grains should not exceed about 3 thousandths of' an inch,- and-the thickness of the coat in actual practice varies from zero toabout 3 to 5 thousandths of an inch.

The cathode tubes are at ground potential and are held in positionj by supporting plates 6|, 62. the ends of -the cathodes projecting into the plates. -Each cathode is `provided with an'anode Wire 64A, 64B, {MQ-64D, 54E, SAG, 34H, llJ passing'coax-ially through' its cathode and insulated therefrom..` The anode wires are held at their ends by frames 55, 65, and are connected electrically in parallel to :the lead wire b1` which passes.' out through, the insulated` bushing 53 in the elfldlllytQLMft" jpl The space withingwhich .the anodes andithe cathodes are disposed' in-the counter is closed, save for the exhaust tubeconnection, In assem bly, a vacuum pump is coupled to this exhaust tube, and the apparatus is pumped out while it is heated to vapproximately C. It is then ushedpseveral times with dried'natural gas and nally after oo oling to room temperature is filled with natural gas at a' pressure of about two inches of mercury. The natural gas contains about 99% methane Vand may be considered substantially pure methane. After the space in which the anodes and cathodes 'are' contained is thus filled, theexhaust tube is crimpedoff vand sealed with soft solden vIf" dcsir'ed the cylinderjcontaining vthe counter andthe fast `neutron sourcemay be shock proofed by enclosing it in a fabric sleeve and rubber tubing combination not shown) tting around the counter and further enclosed within asteel pressure-proof housing, within which the entire' exploring tool illustrated in Fig, 1 is enclosed.

Operation of the counter The counter has an active volume between anode wires and cathode tubes which is roughly co-extensive with the wires. The active volume between any one of the cathode tubes and its axially located anode wire is energized by a radial electric eld of force. y Y, In essence, the counter is a condenser having a certain capacitance -C. An electromotive force V0 (constant but subject to manual control) and a resistance Rare-connected in series with the condenser, i. e. acrossthe counter. If a pair o f4 ions (a negative primary electron and a residual positively charged molecule) appear in the active gas-filled volume, both ions will be accelerated by the electrical eld, and both'will be decelerated on the average by collisionsvwith molecules of the gas. The motion of the two ions is largelyv determined by field intensity and the gas pressure. The eld'is not uniform, so the electron will travel towardthe wire-with increasing accel eration, and the-positive ion will travel toward the cathode tube with decreasing acceleration. In the immediate vicinity of the small wire, thev eld is relatively very strong, and the electron, can acquire relatively great speed (or great err--4 ergy) between two successive collisions.l If' this primary electron acquires suicient energy,` it can ionize by collision. I The primary electron-- and the secondary electron which is formedv can` again ionize by collisions, producing tertiary eleci trons andsoon; v Thus the-primary electronmay produce van avalanche -of electrons the lsize of which increases with increases in V0, andwhich; may number several hundred. This avalanchel arrives speedily at the anode, and coincidentlyl formed positive ions travel more slowly -to-the cathode. j.

The charge motion thus generated within the active volume constitutes an electric current which flows through the resistance R and'causes a drop in potential at the anode. Subsequently,` assuming that there is an interval during which no ions appear in the active volume, the charge will cease to move therein, and the potential of the anode will increaseto its normal value. The

' time required for this recovery will be determined largely by the value of the product RC.

The variation ofA potential at the anode constitutes a negative voltage pulse, the magnitudeY of which depends upon the number of electrons constituting the avalanchearriving at the anode;-

'-'The' avalanche considered above wasV produced byone single primary electron. There is a range of values for Vn at which the size of an avalanche depends upon the number of primary electrons appearing nearly simultaneously in the activev volume. Consequently, for such values of Vo, the size of the voltage pulse atthe anode depends upon the number of primary electrons causing it. Thus the pulse amplitude is proportional to the number of primary electrons and in consequence the apparatus is called a proportional counter.

Detection .of slow neutrons The counter described herein, due to the presence of the activeisotope of boron on thecathode, is sensitive'to slow. neutrons, but is also sensitive to gamma rays or anything else capable of providing primary electrons within the-active vol.- ume. For-the present purposes, however, it is only necessary to consider the primary electrons formed as the result of action within the counter by the slow neutrons and the gamma rays. This is because the number of primary electrons produced within the active volume by theaction ,of fast neutrons is negligible. Y

AV gamma ray V(p h ton) canv project an electron from the solid Vor gaseous matter within the counter, and this electron can produce by collision along its path in the active volume a number of primary electrons. The original-electron'supplied by the gamma ray in the counter may receivelfrom the ray an energy of afmillion elec-Vy tron volts or more, but the design of the counter is suchthat this energy cannot be expended in pro# ducing primary electrons, the dimensions of the cathode tubes and pressure Yand kind of gas being such that only a fraction o f the energy is so xpended, a large proportion of the balance being expended in harmless ionization processes within 'metal at the beginning and end of its path'.- EvenY so,'thecounterY yields a largev number of so-called gamma ray pulses.

.'A'slow neutron produces no direct ionization within the active volume, but slow neutrons introduced into the counter are" captured by the active isotope of boron with resultant emission of an "alpha particle and an energy of several million' electron volts. `The alpha particle has considerably less penetrating power than an electron having'the same energy, principally" because of the largerY relative charge and 'mass of the alpha par-v ticle. The counter is'so dimensionedthat the alpha particle may expend all of its energy with .in the active volume.'v Thus, an alpha particle'- may produce aV proportionally larger number of' primary 'electrons than in the case'of a gamma' ray. This brings about v'an'v increase of the aval" lancheatthe anode." At 'first' glance, then, the rI'Julse'slile''to the slow neutronsshould be bigger Howlv than'the pulses due to the gamma rays. ever, some of the alpha particles lose a 'large fraction'of their energy before emerging from the' boroncarbide coating and so have less energy with which to produce primary electrons within the active volume. The result is that some of the pulses due to slow neutrons have amplitudes no greater than those due to gamma'rays' and, like those-due to Vgamma ray pulses, -are graded 'in'v amplitude Icontinuouslyf'downward to zero'.v

The net result of the foregoing actionwith'- in the proportional oounter'of the invention is to produce from some of the slow neutrons pulses having an amplitude greater than the amplitude of Vany pulses due to ygamma rays and to producev from other slow neutrons pulses having 'ampli- Cil 1-2 tudes 'nojgreater thanthose of pulses obtained from gamma rays. Suppression of the pulses resulting from gamma' rays, therefore, is necesi sarily accompanied by the suppression of some pulses due to slow neutrons. The distribution of neutron pulse sizes', lso means that the boron carbide counter is vnot such that its pulse rate is independent of counter voltage. Nevertheless an amplier of suitable type (for example that of the amplifier-discriminator and AVC channel of Fig. 1) can largely overcome this disadvantage so that over a range exceeding volts the combination of boron carbide counter and amplier may have a ree markably flat plateau, as discussed hereinbefore A slow neutron detector sensitized with boron carbide is' superiorA primarily because of vits long lifel (due in part to the chemical inertness of boron carbide), its relative ease of construction and its relatively high eiciency at low counter voltages. Thus a boron vcai'bide counter has about 10 times the ecienc'y'of one sensitized with boron Afluoride gas, assuming that both are designed vsuitably for welllogging useand to operate at comparable voltages. f

Combined action of source, environment, and

counter if a neutron counter of the type described is lowered into a bore hole, nthe action is as fol-l lows: Fast neutrons and gamma'rays emitted from the-source stream outwardly through the counter and penetrate through the exploration tool`v housing, pass through any well fluid or casing present and into the adjacent rock formation. The counter detects many gamma rays-but only a negligible amount of fast neutrons. The fast neutrons are slowed down by nuclear collisions in the material surrounding the place of emission and eventually are captured by the nuclei. Some of the fast neutrons following colf' lision and slowing down are scattered back to' the counter as detectable'slow neutrons.

Other than empirically, the back scattering from a given rock formation is dilcult to determine because elements vary greatly in the effect lwhich thay have in scattering and cap-Y for fast neutrons. Hydrogen is perhaps more eiective than any other element in slowing down?.

fastV neutrons, but it is also a goodrabsorberftor slowl neutrons.

the back scattering of slow neutrons from various rock formations diiers sufficiently to permit its measurement to be' a useful Vindex in logging.

Thev counter source and fdiscriminator de-v scribed yield more 'than a' thousand "neutron pulses'per second at the recording end of 11,006"r feet of Ycable employed in logging operations. The

variations in counting rate, sov far observed' inunconsolidated sediments of a laboratory ltest hole, are much less than this, say a few hundred per second; i. e. from one formation to the next, the counting rate may vary by only 10 or' Moreover the average lifetime'- of a neutron varies greatly. Net effect ofa'f' given formation is, therefore,v diflicult to l .eal culate in advance of actual test. Nevertheless,"

13 20%, and useful information may be represented by even smaller differences in counting rate. However, the equipment is such that the total counting rate is measured with a sufficient high order of accuracy to reliably disclose significant minor differences.

This is illustrated by Fig. 2 Whereon fast neutron-slow neutron logs made in accordance with the invention are disposed side by side with a conventional gammal ray log, a conventional self-potential electrical log and a conventional electrical resistivity log made in the same hole. The fast neutron-slow neutron logs made in accoidance with the invention are easily distinguishable from the other logs and give different details.

The left hand fast neutron-slow neutron log was made before cementing the well at approximately 1950 feet. The next two logs represent two runs made with the apparatus of the invention following cementing. There is a remarkable correspondence between the two runs, showing that the apparatus gives reproducible results and moreover there is a marked decrease in neutron intensity exactly at the top of the cement.

Both of the electrical logs which are reproduced at the extreme right hand of Fig. 2 were run before the casing was cemented. The gamma ray log was run after the casing was cemented. Consequently, the other logs are not strictly comparable to the fast neutron-slow neutron logs and are reproduced only to show the difference in character of the several types.

I claim:

1. In a logging device, the combination which comprises a logging head containing a source of fast neutrons and gamma rays, a proportional counter sensitive to slow neutrons and gamma rays and capable of producing pulses of greater amplitude from slow neutrons than from gamma rays, an ampliiier-discriminator circuit adapted to suppress pulses of the gamma ray amplitude and to lleave unsuppressed pulses of neutron amplitude, a counter voltage supply, and an automatic volume control for controlling the amplitude-discriminator circuit, said automatic volume control being responsive to the gamma rays being detected by the proportional counter, but substantially unresponsive to the slow neutrons detected by the counter.

2. In a llogging device the combination which comprises a logging head containing a source of fast neutrons and gamma rays, a proportional counter sensitive to slow neutrons and gamma rays and capable of producing pulses of greater amplitude from slow neutrons than from the gamma rays, means for suppressing pulses of the amplitude of those produced from the gamma rays, an automatic volume control for controlling the suppressing means, said automatic volume control being responsive to the gamma rays detected by the proportional counter but unresponsive to the slow neutrons detected by the counter, and means for measuring the unsuppressed pulses.

3. Apparatus according to claim 2 in which the automatic volume vcontrol is disposed in the logging head.

4. In a logging device the combination which comprises a logging head containing a source of fast neutrons and gamma rays and a proportional counter sensitive to slow neutrons and gamma rays and capable of producing pulses of greater amplitude from slow neutrons than from gamma rays, an amplitude-discriminator circuit adapted to suppress pulses of the gamma rays and to leave unsuppressed pulses of neutron amplitude, said amplifier-discriminator circuit comprising a multi-stage amplifier including a first and second amplifying tube, a third tube having a remote cut-ofi characteristic, a fourth tube, means biasing the fourth tube beyond cut-01T at a level such that al-l pulses smaller than a certain size are suppressed, a fifth tube connected to receive the unsuppressed pulses from the fourth tube, and a power tube operated beyond cut-off, the amplified pulses being fed from the fth tube to the grid of the power tube, and an automatic volume control for`controlling the amplier-discriminator circuit, said automatic volume control being responsive to the gamma rays detected by the proportional counter but substantially unresponsive to the slow neutrons detected by the counter. g

5. In a logging device the combination which comprises a logging head containing a source of fast neutrons and gamma rays, a proportional counter sensitive to slow neutrons and gamma rays and capable of producing pulses of greater amplitude from slow neutrons than gamma rays, an amplifier-discriminator circuit adapted to suppress pulses of the gamma ray amplitude and to leave unsuppressed pulses of neutron amplitude, and an automatic volume control comprising a first amplification stage, means for feeding unequalized pulses from the amplifier-discriminator circuit to the first amplification stage, a rst rectier tube for receiving the amplified pulses for the rst amplification stage, an integrating circuit comprising a resistance capacitance network for integrating the pulses from the first rectifier tube, a second amplification stage, means for feeding the unsuppressed pulses from the amplier-discriminator circuit to the second amplification stage, a second rectifier tube connected to receive pulses from the second amplication stage and means for feeding the net voltage from the second rectier to the amplifier-discriminator circuit.

KENNETH C. CRUMRINE.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS 1946, D. Van Nostrand, pages 196, 197. 

1. IN A LOGGING DEVICE, THE COMBINATION WHICH COMPRISES A LOGGING HEAD CONTAINING A SOURCE OF FAST NEUTRONS AND GAMMA RAYS, A PROPORTIONAL COUNTER SENSITIVE TO SLOW NEUTRONS AND GAMMA RAYS AND CAPABLE OF PRODUCING PULSES OF GREATER AMPLITUDE FROM SLOW NEUTRONS THAN FROM GAMMA RAYS, AND AMPLIFIER-DISCRIMINATOR CIRCUIT ADAPTED TO SUPPRESS PULSES OF THE GAMMA RAY AMPLITUDE AND TO LEAVE UNSUPPRESSED PULSES OF NEUTRON AM- 